Electrochemical sensing circuits

ABSTRACT

An electrochemical cell sensing circuit for an electrochemical cell having a working electrode, a counter electrode and a reference electrode in an electrolyte, which in use, when a gas to be analysed is introduced into the cell, generating a current between the counter electrode and the working electrode, and a potential at a position in the electrolyte is sensed by the reference electrode. The circuit comprising power supply means ( 12, 13, 14, 15, 16 , R 4 , R 5 , D 1 ) for applying an offset voltage to the counter electrode (c) relative to the working electrode (w), an amplifier means (A 3 , A 5 ) for monitoring the voltage difference between the reference electrode (r) and the working electrode (w) and operable in use to feed back a current to the working electrode (w) through a feed back loop, and thereby tend to maintain the working electrode (w) at substantially the same potential as the reference electrode (r), the circuit includes measuring means ( 15, 16, 17 , S 1 , R 3 , C 2 , A 3 ) for measuring the current feed back by the amplifier (A 2 , A 3 ) to the working electrode (w) as a measure of the cell current flowing between the working electrode (w) and the counter electrode (c).

BACKGROUND OF THE INVENTION

This invention relates to electrochemical sensing circuits and inparticular to electrical circuits which sense cell current flow from athree terminal electrochemical cell.

Three terminal electrochemical cells are used for a variety of gasmonitors and comprise a cell in which a gas to be analysed is introducedand three spaced apart electrodes. The three electrodes comprise a mainpair across which the cell current is generated and a referenceelectrode which enables a potential at a predetermined point in the cellelectrolyte measured. The cell current is proportional to theconcentration of the compound or element being sensed by the cell, whichmay, for example, be carbon monoxide.

Known three-terminal electrochemical cells can be stabilised using thecircuit shown in FIG. 1. In order to stabilise the cell, the “working”and “reference” electrodes, labelled “W” and “R” respectively must bebrought to the same electrical potential. No current is taken from thereference electrode. Instead, current is injected into the counterelectrode, labelled “C”, by the amplifier Al until both the referenceand working electrodes, R and W respectively, are the same potential.The current which flows in both the counter and working electrodes isthe cell current and due to the internal operation of the cell this isproportional to the concentration of the compound being sensed by thecell.

Referring to FIG. 1, the Amplifier, A1, maintains the referenceelectrode at 0V by feeding back current to the counter electrode.Amplifier A2 maintains the working electrode at 0V, since the negativeinput of amplifier A2 is at 0V. The cell current is driven by amplifierA1 but is sensed by amplifier A2, because the cell current passesthrough resistor R2 to develop the voltage V out.

A disadvantage of the prior known circuit shown in FIG. 1 is that it isprone to oscillation, because the virtual earth impedance of eachamplifier appears as part of a feedback path of the other amplifier.This can lead to oscillation at high frequencies, where the virtualearth impedances are not well defined.

A second disadvantage, for low cost microcontroller-based applications,is that the output V out is an analogue voltage which must go through ananalogue to digital conversion before it can be processed digitally.

A third disadvantage is that, whereas V out is normally positive whengas is being sensed, the counter electrode charges negatively, requiringthe output of amplifier A1 to go negative. Therefore the circuit shownin FIG. 1 requires both positive and negative supplies (shown as V+ andV−).

A further cell which uses a potentiostat-type circuit is described inU.S. Pat. No. 4,048,041 (U.S. Army). The electronic circuit controlsvoltage potential applied to working electrodes of a three-electrodeelectrochemical cell. The electrochemical cell is incorporated into asensor which operates by measuring the difference current between thecell's grounded anode and a negatively pulsed cathode. This circuit isquite complex, and requires both positive and negative supplies.

BRIEF SUMMARY OF INVENTION

An object of the present invention is to provide a simplified sensingcircuit in which at least one of the above mentioned disadvantages isovercome, and which can be powered by an isolated DC supply, such as abattery.

According to one aspect of the present invention there is provided anelectrochemical cell sensing circuit comprising an electrochemical cellhaving a working electrode, a counter electrode and a referenceelectrode in an electrolyte, the cell being constructed such that inuse, when a gas to be analysed is introduced into the cell, a currentflows between the counter electrode and the working electrode, and apotential at a position in the electrolyte is sensed by the referenceelectrode, the circuit further comprising power supply means forapplying an offset voltage to the counter electrode relative to theworking electrode, an amplifier means for monitoring the voltagedifference between the reference electrode and the working electrode andoperable in use to feed back a current to the working electrode througha feed back loop and thereby tend to maintain the working electrode atsubstantially the same potential as the reference electrode, andmeasuring means for measuring the current feed back by the amplifier tothe working electrode as a measure of the cell current flowing betweenthe working electrode and the counter electrode.

Preferably the measuring means comprises a resistor means in seriesconnection between an output of the amplifier and the working electrode,and means are provided for measuring the voltage generated across theresistor.

Alternatively the measuring means comprises a capacitor connected inseries between an output of the amplifier and the working electrode, aswitch means connected in parallel across the capacitor, said switchbeing operable in a closed position to short out the capacitor and in anopen position to allow the capacitor to be charged by the current feedback to the working electrode by the amplifier and a comparator adaptedto receive, at a first input, the output of the amplifier, and at asecond input, a reference voltage Vr, said comparator being operable tocompare the first and second inputs and produce an output signalindicative of the cell current when said switch is open.

Preferably the power supply means comprises a DC supply which applies apositive potential to the counter electrode.

Preferably the amplifier is connected between a second DC supply whichis isolated from the DC supply which applies to the positive potentialto the counter electrode.

Preferably the said power supply means comprises a DC supply whichapplies a positive potential to the counter electrode and the comparatorand amplifier are connected between a second DC supply which is isolatedfrom the DC supply which applies the positive potential to the counterelectrode.

The output signal from the comparator may be a digital signal. A meansmay be provided to inject a pulse in the supply to the counter electrodeto provide a means of testing the correct operation of the circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Two embodiments of the present invention will now be described, by wayof example, with reference to the accompanying drawings, in which:

FIG. 1 illustrates a known electrochemical sensing circuit and isdiscussed above.

FIG. 2 illustrates one embodiment of the present invention, and

FIG. 3 illustrates a second embodiment of the present invention forconverting the cell output into a timing signal.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, the electrochemical cell 10 is of conventionalconstruction and comprises a chamber, or cavity 11 into which a gas tobe monitored is introduced. Three spaced electrodes W, C, R are locatedin the cavity 11. Electrode R constitutes a reference electrode and isconnected to an input 12 of an operational amplifier A2. A second input13 of amplifier is connected to the electrode W. The output 14 of theamplifier A2 is connected to electrode W through a feedback loop whichincludes resistor R3. The counter electrode, C, is connected to areference voltage generated by resistor R4 and diode D1 which isconnected to the 0V line.

The amplifier A2 feeds back current through resistor R3 to maintain theworking and reference electrodes, W and R respectively, at the samepotential.

When gas is sensed by the cell 10, the output of amplifier A2 goespositive to deliver a positive current into the working electrode W, andat the same time, the electrode W charges positive with respect to thecounter electrode C. These two effects ensure that the output amplifierA2 remains positive relative to the 0V line at all times.

The voltage across R3 is an accurate output which is proportional to gasconcentration in the cell 10. If the +V supply is provided by anisolated battery (not shown), the terminals 15, 16 of the resistor R3can be taken as the output with one terminal connected to an externalisolated ground. Where accuracy is less important, it may be sufficientto sense the output of amplifier A2 relative to the 0V line, but theoutput voltage of amplifier A2 includes a component due to the offsetvoltage between the working electrode, W, and the counter electrode C.The offset is normally a very small fraction of a volt.

The offset voltage on the counter electrode, C, is generated by resistorR4 and diode D1 and protects those cells which would otherwise have anin-built tendency for the counter electrode to be positive relative tothe working electrode. The offset voltage also allows for exposure togases which would otherwise spuriously provoke a reverse response fromthe cell. If the cell is designed to have an inbuilt positive bias fromthe counter electrode, C, to the working electrode, W, it may not benecessary to impose an offset voltage on counter electrode C.

An experimental circuit based on FIG. 1 used a Maxim MAX 406 amplifierand R4 provided 1 μA into D1 from a 6 volt battery. R3 was 12 kΣ, togive the cell an output sensitivity of 1 mV/ppm of carbon monoxidesensed by the cell under test. The total current consumption was 3 μAwhich would provide a battery life of many years.

FIG. 3 shows a further embodiment of the invention in which the outputfrom the cell is converted into a timing signal. In FIG. 3 similarcomponents to those shown in FIG. 2 are given the same referencenumeral.

Referring in greater detail to FIG. 3 the amplifier A3 operates in verymuch the same way as amplifier A2 of FIG. 2 except that the cellcurrent, instead of flowing through resistor R3, flows through switch S1when the switch is closed, or through capacitor C2 when the switch S1 isopen.

The output 16 of the amplifier A3 is connected to one input of acomparator 17 and a reference voltage Vr is applied to a second input ofthe comparator 17. The reference voltage may be provided by a separatecircuit or could be a tapping on the resistor R5.

To measure the cell current, a switch S1 is first closed therebyshorting out capacitor C2. A timer (not shown) is started, and switch S1is opened. The capacitor C2 then charges positively at a rateproportional to the cell current. The comparator 17 switches when theoutput voltage of the amplifier A3 equals the reference voltage Vr, thusproducing a timing signal 18 representative of the cell current.

A sensor based on the circuit of FIG. 3 will suffer minor errors due tothe voltage offset between the working electrode, W, and the counterelectrode, C. If greater accuracy is required the reference voltage Vrcan be made to vary with the working electrode voltage as buffered, forexample, by an emitter follower.

What is claimed is:
 1. An electromechanical cell sensing circuitcomprising: an electromechanical cell having a working electrode, acounter electrode, a reference electrode and an electrolyte, saidreference electrode sensing a potential at a position in saidelectrolyte when an introduction of gas to be analysed into saidelectromechanical cell provides a current flow between said counterelectrode and said working electrode; a power supply having a pair ofvoltage rails at different potentials; biasing means for offsetting apotential of said counter electrode from one of said voltage rails; anamplifier to monitor a voltage difference between said referenceelectrode and said working electrode, and to apply a feedback current tosaid working electrode through a feedback loop to maintain said workingelectrode at substantially a same potential as said reference electrode;and a measurement circuit to monitor said feedback current as a measureof said current between said working electrode and said counterelectrode.
 2. The electrochemical cell sensing circuit according toclaim 1, wherein said measurement circuit comprises: a resistor betweenan output of said amplifier and said working electrode; and at least oneterminal allowing a voltage generated across said resistor to bemeasured therefrom.
 3. The electrochemical cell sensing circuitaccording to claim 1, wherein said power supply comprises: a firstdirect current supply applying a positive potential to said counterelectrode.
 4. The electrochemical cell sensing circuit according toclaim 3, wherein: said amplifier connects to a second direct currentsupply isolated from said first direct current supply.
 5. Theelectrochemical cell sensing circuit according to claim 1, furthercomprising: testing means for testing a correct operation of saidelectrochemical sensing circuit by injecting a test pulse into saidcounter electrode.
 6. An electromechanical cell sensing circuitcomprising: an electromechanical cell having a working electrode, acounter electrode, a reference electrode and an electrolyte, saidreference electrode sensing a potential at a position in saidelectrolyte when an introduction of gas to be analysed into saidelectromechanical cell provides a current flow between said counterelectrode and said working electrode; a power supply applying an offsetvoltage to said counter electrode relative to said working electrode; anamplifier to monitor a voltage difference between said referenceelectrode and said working electrode, and to apply a feedback current tosaid working electrode through a feedback loop to maintain said workingelectrode at substantially a same potential as said reference electrode;a measurement circuit to monitor said feedback current as a measure ofsaid current between said working electrode and said counter electrode;a capacitor between an output of said amplifier means and said workingelectrode; a switch in parallel across said capacitor, said switchshorting out said capacitor when said switch is in a closed position,and said switch allowing said feedback current to charge said capacitorwhen said switch is in an open position; and a comparator adapted toreceive said output of said amplifier at a first input, and a referencevoltage at a second input, said comparator comparing said first inputand said second input to produce an output signal indicative of saidcell current when said switch is in said open position.
 7. Theelectrochemical cell sensing circuit according to claim 6, wherein saidpower supply comprises: a first direct current supply applying apositive potential to said counter electrode; and a second directcurrent supply supplying power to said comparator and said amplifier,said second direct current supply being isolated from said first directcurrent supply.
 8. The electrochemical cell sensing circuit according toclaim 7, wherein: said output signal is a digital signal.